This invention relates to a method for heat treatment of pulverous material in an apparatus comprising a preheater, e.g. a suspension preheater, a suspension calciner with two chambers and a cooler, e.g. a suspension cooler, by which method the material is preheated in the preheater by means of exit gas from the heat treatment, being burnt in the calciner and cooled in the cooler by air being used as combustion air in the calciner.
Lime burning in a suspension calciner, a so-called Gas Suspension Calciner, of the above type is known from the British Patent No. 1,463,124.
A method of the above kind and the appertaining apparatus is known from the description of the German patent DE 25 50 469. According to this patent the usage of two chambers in the calciner will provide optimum burning conditions when the apparatus is used for burning different materials. All material which is burnt in the apparatus is led through both chambers of the calciner.
The operating principle of this known apparatus corresponds to an apparatus, of which the present invention is a further development, and which is described in detail in the magazine ZEMENT-KALK-GIPS, vol. 10/1980, pages 4935-497, and also shown in the enclosed FIG. 1.
The mentioned magazine article describes a typical industrial plant for lime burning, comprising a 3-stage cyclone preheater in which the raw material is preheated by means of the exit gas from the calciner which is equipped with a separation cyclone in which the burnt material is separated from the exit gas and transported to a 2-stage cyclone cooler.
However, one characteristic of a method and plant of this type is that the finish-burnt lime is normally highly reactive. Although such high reactivity may be advantageous in certain cases, it is a disadvantage in other cases.
For instance, burnt lime is used to produce milk of lime which is a suspension of calcium hydroxide particles (Ca(OH).sub.2), in water, used i.e. for environment cleaning purposes. Milk of lime produced from highly reactive lime will often become lumpy, because it contains large agglomerates of Ca(OH).sub.2 particles instead of many small particles, paradoxically resulting in a less reactive milk of lime, therefore being less applicable for certain purposes.
It has been possible, over a short period, to produce less reactive lime by means of a suspension calciner; but it was not possible to maintain an undisturbed, continuous production, because this requires a higher calcining temperature than the normal 900.degree.-1100.degree. C. in the calciner. Due to this higher temperature the separation cyclone after the calciner will also operate at a higher temperature, entailing that the lime product may become rather sticky and resulting in an accumulation of material in the separation cyclone and clogging of same and a consequential negative influence on production.
It should be mentioned that there are other methods and apparatus for lime burning by which the finished product becomes considerably less reactive, e.g. by burning the lime in rotary kilns or in shaft kilns. Such plants, however, especially rotary kilns, have the disadvantage that the installation costs are considerably higher and the heat economy is considerably lower than for a suspension plant of the type described. Particularly in relation to a shaft kiln, a suspension plant is far easier to start and stop and it is also easier to control temperatures and surplus air and to maintain a constant low loss on ignition of the product. Moreover, a suspension plant is more suitable than other plants to handle, as raw material, lime sand and finer waste products of limestone and to process materials not at all suitable for other types of plants.